Post-deflection-focus cathoderay tube



Aug. 20, 1968 s. H. KAPLAN POST-DEFLECTION-FOCUS CATHODE-RAY TUBE Filed Aug. 10, 1966 J awn 6 1 T. m J

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United States Patent 3,398,309 POST-DEFLECTION-FOCUS CATHODE- RAY TUBE Sam H. Kaplan, Chicago, Ill., assignor to The Rauland Corporation, Chicago, Ill., a corporation of Illinois Filed Aug. 10, 1966, Ser. No. 571,533 9 Claims. (Cl. 313-85) The present invention is directed to a post-deflectionfocus cathode-ray tube by which is meant a tube including a focusing system positioned between the beam deflection region and the image screen. While of general application to monochrome as well as to color image reproducing devices, it is especially suited for color cathode-ray tubes of the type that employ a color selection electrode and it will be described in that environment.

Color cathode-ray tubes as presently constructed have an image screen in which there are interlaced or interleaved series of phosphor deposits. One series comprises elemental screen areas which, for a three-color system, emit red light in response to excitation by an impinging electron beam. The other interlaced series are usually similar in configuration but differ from the first in that one of the remaining series emits green while the other emits blue light upon electron excitation. Where the phosphors have equal efficiencies, the deposits of each of the three series have approximately the same size although a larger elemental size may be employed for any of the phosphors that may be less eflicient than the remaining ones; conversely unequal currents may be used to compensate for different phosphor efficiency. This arrangement of interlaced series of phosphor elements may have specifically different patterns.

By way of illustration, a very popular form of color cathode-ray tube of this general description has each phosphor deposit in the form of a dot and, for a threecolor system the dots are arranged to define a multiplicity of clusters or triads including in each triad a red, a green and a blue phosphor element. Another well-known variety has the color phosphors in the form of a repeating array of strips extending in the direction of field scansion.

Image reproduction in simulated natural color may be accomplished with either type tube by the use of a single electron beam or three electron beams. In either case, a color selection electrode is used to make certain that the phosphor elements are excited by the scanning beam in proper relation to the color information of the received signal.

The most popular structure used today is the shadow mask or aperture mask tube in which the color selection electrode is positioned close to the screen On the side facing an array of three electron guns. The electrode has apertures which correspond in number and configuration to the members of a given series of phosphor elements and the beams are controlled to emerge through a given aperture of the mask at the necessary angle so that each beam impinges only upon its assigned phosphor element.

It is well known that much of the beam strikes the aperture mask and makes no contribution to brightness. It is for this reason, among others, that post-deflection focusing has been proposed to accomplish focusing of the beam as it emerges from the shadow mask with a consequent increase in brightness. The arrangements heretofore proposed have exhibited certain defects which have discouraged their use. For example, if the color selection mask is made to also serve as a focus lens member by being at a lower potential than the screen, there is a great danger that secondary electrons will be driven .off the color selection mask and be accelerated to the screen. Of themselves, they give rise to an undesired halo effect on the screen and they may also cause a further emission 3,398,399 Patented Aug. 20, 1968 of what are known as reflected primaries at the screen. The reflected primaries also return to the screen and degrade both color and contrast.

Another difficulty that has been experienced, particularly where the focus lens makes use of a series of electrodes, is the tortuous path that the electron beam travels in reaching the screen. Where this occurs, there are grave difiiculties in effecting appropriate beam landings on the elemental phosphor areas.

Efforts to .overcome these difficulties have resulted in what is known as a structured mask which has two conductive aperture masks aligned ahead of the screen and energized so that the mask closer to the cathode is at the higher potential. With this structure there is some relief from the effect of secondaries which now tend to be collected by the mask element maintained at the higher potential. It has been found however that the voltage differences required for focusing are exceedingly high, being at least of the order of three to one. As a practical matter, the requirement of such a large potential difference in the focusing system is highly undesirable because of complications to the power supply and because of dangers due to arcing.

Accordingly, it is an object of the invention to provide an improved post-deflection-focus cathode-ray tube.

It is a specific object of the invention to provide a postdeflection-focus cathode-ray tube which avoids one or more of the described difficulties of prior structures.

It is a specific object of the invention to provide a postdeflection-focus cathode-ray tube, especially for use in color image reproducers, characterized by the fact that the color selection electrode is constructed to approximate a uni-potential lens focusing system.

The tube to which the invention is applicable, as stated above, is of the post-deflection-focus type having an image screen at one end and at least one electron gun at the .opposite end for developing and for directing an electron beam along a path and through a beam deflection region toward the screen. The improvement of the invention comprises a unitary electrode structure positioned across the beam path between the screen and the deflection region having a principal conductive member with a multiplicity of electron permeable portions and intervening electron impervious portions. A pair of auxiliary conductors are superposed over but insulated from both faces of the electron impervious portions .of the principal member. There are means for supplying substantially the same operating potential to the auxiliary members and means for applying a different potential, preferably lower, to the principal member to form at each of the electron permeable portions of the electron structure a uni-potential lens for focusing the beam on the screen.

While the electrode structure may be in the form of a wire grid, it preferably is an aperture mask having a conductive core with an overlying layer of insulation on each face and a conductive coating superposed on each layer of insulation. The three conductors, consisting of the two conductive layers and the conductive core, comprise the electrodes of a unipotent-ial lens which is operated with the outer electrons at the same potential and the inner one at a different, usually a lower potential. An exceedingly simple structure is possible if the core is made of aluminum anodized to provide a surface of insulation upon its opposed faces and upon which an aluminum layer is deposited by vacuum evaporation to serve as the outer elements of the lens system.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with a further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in the several figures of which like reference numerals identify like elements, and in which:

FIGURE 1 is a representation of a post-deflectionfocus cathode-ray tube embodying the present invention; and

FIGURE 2 is an enlarged fragmentary view of the color selection electrode of the tube represented in FIG- URE 1.

The cathode-ray tube of FIGURE 1 is for the production of images in simulated natural color and has a glass envelope 21 terminated 'at one end in a faceplate 22 which serves as an image screen. Reference character 23 indicates the deposit of phosphor elements on the internal surface of faceplate 22. Since the nature of the screen is of no particular concern to the invention, it Will be assumed for convenience that the screen is of the mosaic type having a multiplicity of phosphor dot triads, each triad having a red, green and blue elemental area of phosphor as is characteristic of tri-color shadow mask tubes presently in commercial use. Over the phosphors is applied the usual layer 24 of conductive and light-reflecting material, such as aluminum. Of course, this layer is pervious to electrons as required to achieve excitation of the phosphor. At the opposite end, envelope 21 terminates in a neck portion which houses an array 25 of electron guns. The guns are similar to one another and are of well known construction, individually serving to develop and direct an electron beam 'along a predetermined beam path and through a beam deflection region toward screen 22. Since the gun structure is well known and constitutes no part of the persent invention, it has not been shown in detail. Of course, it will include the usual convergence assembly and will have contact elements 26 which make contact with the customary aquadag coating 27 on the conical section of the envelope which connects the neck to the screen portion. The deflection region of the envelope is the portion enclosed by a deflection yoke 28 having line and field scanning coils suitably energized to establish deflection fields for causing the electron beams to scan screen 22 in a series of fields of parallel lines in a manner well understood in the art. Adjacent deflection yoke 28 is the convergence yoke assembly 28a which is energized by suitably shaped signals at line and field frequencies to accomplish dynamic convergence of the three electron beams as they are caused to course over screen 22.

Interposed between screen 22 on the one hand and the beam deflection region adjacent yoke 28 on the other is an electrode structure 30 which, for the present, may be considered as simply the usual aperture or shadow mask customarily employed in a tube of the type under consideration. It has a multiplicity of apertures of similar configuration to the elemental phosphor dots of screen 23 and each aperture is aligned with a particular one of the dot triads. The three guns of cluster 25 are mechanically converged so that the beams issued therefrom pass through apertures of mask 30 and each beam strikes only the color phosphor dots to which it has been assigned. Obviously, a power supply is required for applying energizing potentials to the electrode system of the tube and other signal sources are necessary for energizing yokes 28 and 28a and for modulating the electron beams with luminance and chrominance information. Here again, these components are well known and, of themselves, constitute no part of the invention; therefore, they have been omitted from the drawing.

In operation, each of the three beams from source 25 is modulated by luminance information and is directed through the deflection field of the tube toward screen 22, reaching the screen by passing through 'apertures of mask 30. The deflection fields established by yoke 28 cause these beams to scan the screen in a series of fields of parallel lines and during the scansion, the convergence field established by yoke 28a maintains the beams converged even though they are deflected to travel from side to side and top to bottom of screen 22 in synthesizing an image field. At the same time the three beams from source 25 are individually controlled in accordance with the chroma information of their assigned color fields. In this way, the electron beam is modulated with luminance and chrominance information and synthesizes an image in simulated natural color on screen 22 in well known fashion.

Consideration will now be given to the post-deflection focusing feature which constitutes the improvement of the subject invention to a tube of the type represented in FIGURE 1. The focusing system of the invention comprises electrode structure 30 which, as previously explained, is also the color-selection electrode of the tube. It is positioned across the beam paths between screen 22 and the deflection region encompassed by yoke 28. Structurally, it has a principal conductive member with a multiplicity of electrode permeable portions and intervening electron impervious portions. As stated, this member generally is in the form of a conductive plate with holes for passing an electron beam to screen 22. In the enlarged fragmentary view of FIGURE 2 the principal conductive member is designated 31. There are a pair of auxiliary conductive members 32, 32 superposed over but insulated from both faces of the electron impervious portions of member 30. This structural arrangement is most easily obtained by providing the opposing faces of member 31 with layers 33, 33 of insulating material and having members 32 take the form of conductive layers deposited over the insulation layers. By way of illustration, the core or principal member 31 may be a structure of aluminum which is anodized to have insulating layers 33 of aluminum oxide on each face. If desired, the aluminum member may then be baked to seal the insulating layers and avoid objectionable porosity after which aluminum may be vacuum evaporated to form conductive layers 32 over the insulation 33, 33.

It is well known that aluminum coated steel is available and the color-selection-lens electrode may be formed of that material. In this case, the aluminum overcoating is anodized to form insulation layers 33 which are then coated with layers 32 of aluminum. As clearly illustrated in FIGURE 2, the final structure has three aligned conductive electrodes with intervening layers of insulation. All of these elements have aligned apertures which define the paths through which the electron beams are admitted to screen 2.

In order to obtain the desired focusing action, there are means for applying substantially the same operating potential to auxiliary conductive members 32, 32. Preferably this is simply a connection from layers 32, 32 to the aquadag 27 formed on the inner surface of glass envelope 21. The aquadag usually extends at least from the beam deflection region adjacent yoke 28 along the conical section toward screen 22 and is in electrical contact with conductive layer 24 of the screen. Finally there are means for applying an operating potential to the principal conductive member 31 which is different from that applied to electrodes 32. The central electrode 31 may be higher or lower in potential than auxiliary electrodes 32 but it is preferable that it be established at a lower potential since this simplifies the power supply. As shown, a terminal 34 is brought out from the envelope to permit the application of an operating potential to electrode 31. Obviously, the lead which penetrates envelope 21 to accommodate terminal 34 must be insulated from the aquadag. This is most readily accomplished by having the area immediately surrounding the feedthrough of lead conductor 34 free and clear of the aquadag. With electrodes 32, 32 at the same operating potential and electrode 31 at a different potential they collectively constitute at each aperture of mask 30 a unipotential lens and the strength of the lens as determined in known manner by the potential difference between its inner and outer electrodes to focus the beams on screen 22. This is indicated in FIGURE 2 where the unfocused beam is shown approaching from the left and emerging as a focused beam to the right of the lens structure. Where the final or ultor voltage of the tube is of the order of 25 kilovolts, this is the potential of lens elements 32, 32. If lens element 31' is at a potential difference of approximately. 3200 volts, that is, it has a potential of approximately 21,800 volts, the focusing is effective to reduce the beam diameter at screen 22 to half that of the diameter as the beam arrives at lens 30 and thereby establish a brightness gain at the screen of four times. In'practice a brightness gain of three is adequate and this may be achieved if the voltage difference of the central electrode relative to the outer electrodes is approximately 2300 volts. These illustrative figures apply to a lens structure having a focal length of 50 diameters and in which the thickness of electrode 31 is approximately six mills and the thickness of insulating layers 33 is also 6 mills each.

It is the usual practice to form the apertures of the shadow mask by etching and that same procedure may be adopted with respect to electrode structure 30. If the starting or core material 31 is aluminum, it is first anodized to provide insulation layers 33 of aluminum oxide on its opposed faces and thereafter the aluminum is vacuum evaporated to a depth of about 0.1 mil on each layer of insulation to form electrodes 32, 32. After this has been accomplished, the electrode structure may be etched in the usual way to provide the apertures of the shadow mask.

As shown in FIGURE 2, the holes through the mask structure taper outwardly from the side that faces the electron guns. The angle of taper, as understood in the art, exceeds the angle of incidence of the beam so that the beam does not strike the side of the aperture. For example, with a tube having a i45 deflection angle, the angle of incidence of the beam at the edge of the shadow mask is approximately 23 and in such a case the angle of taper of the mask aperture must be in excess of 23.

The described structure has distinct advantages over the prior art. For example the mask structure is a unitary monolithic type rather than three separate aligned masks which are a practical impossibility. It will be observed that the regions of the tube on opposite sides of the mask are field-free spaces and therefore there are no wide excursions of the electron beam attributable to the focusing system. This minimizes the difliculty of appropriate beam landing that is exhibited by prior post-deflection-focus arrangements. The problem of high voltage breakdown inherent in previous structures which have voltage ratios in the lens system of three to one is also greatly reduced because of the drastic reduction in potential difierence required for the electrodes of the lens system. The principal consideration as to voltage breakdown in the subject arrangement is that the insulating layers 32 must be adequate to sustain the voltage difference of the lens electrodes. The phenomenon of secondary emission is not aggravated in the described structure as is the case in prior structures having electrodes in the lens system which produce strong accelerating fields. For the described structure, the problem of secondary emission is about the same as that experienced with shadow mask tubes in commercial use which do not have post deflection focusing. Moreover, because of the field-free space feature, such secondary electrons as may be produced travel in straight lines and in random direction rather than being directly accelerated to the screen. So far as reflected primaries are concerned, there is relief in that no fields are present to return them to the screen and aggravate halo conditions.

Since the lens structure is capable of material focusing of the beam, it is possible to make the apertures of the shadow mask of larger size while still preserving optimum operating conditions. Obviously there is a limit to the size of the apertures in the mask because as the size is increased the structure tends to become mechanically weak. It is believed that the structure here proposed permits increasing mask aperture size from the customary 12 mils to at least 20 mils in a tube arrangement which has a brightness gain attributable to post deflecting focusing of approximately three times.

While a particular embodiment of the invention has been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. In a post deflection focus cathode ray tube having an envelope with a conical section luminating in an image screen at one end and having at least one electron gun at the opposite end for developing and directing an electron beam along a predetermined path and through a beam deflection region toward said image screen, the improvement which comprises:

a unitary electrode structure positioned across said beam path between said screen and said deflection region having a principal conductive member with a multiplicity of electron permeable portions and intervening electron impervious portions;

a pair of auxiliary conductive members superposed over but insulated from both faces of said electron impervious portions of said principal member;

means for applying operating potentials to said auxiliary members;

and means for applying a different operating potential to said principal member to form at each of said electron permeable portions a lens for focusing said beam on said screen.

2. A post deflection focus cathode ray tube in accordance with claim 1 in which both faces of said electron impervious portions of said principal member have a coating of insulating materal and in which said auxiliary members comprise a conductive layer deposited on said insulating coating.

3. A post deflection focus cathode ray tube in accordance with claim 1 in which both faces of said electron impervious portions of said principal member have a layer of aluminum oxide and in which said auxiliary members are layers of aluminum deposited upon said aluminum oxide.

4. A post deflection focus cathode ray tube in accordance with claim 1 in which the operating potential of said principal member is less than that of said auxiliary members.

5. A post deflection focus color cathode ray tube in accordance with claim 1 in which said electrode structure is the color selection electrode of the tube.

6. A post deflection focus color cathode ray tube in accordance with claim 1 in which said conical section, said image screen and said auxiliary members are all at the same operating potential.

7. In a post deflection focus color cathode ray tube of the shadow mask type having an image screen at one end with a multiplicity of elemental phosphor deposits and further having at least one electron gun at the opposite end for developing and directing an electron beam along a predetermined path and through a beam deflection region toward said image screen, the improvement which comprises:

a unitary color selection electrode structure positioned across said beam path between said screen and said deflection region having a principal conducting member with a corresponding multiplicity of apertures in alignment with said deposits of said image screen and further having a layer of insulating material on each face thereof,

a layer of conductive material deposited on each of said insulating layers serving as auxiliary conductive members likewise having a corresponding multiplicance with claim 7 in which the space on either side of said unipotential lenses is essentially a field free space.

9. A post deflection color cathode ray tube in accordance with claim 7 in which said color selection electrode is aluminum coated steel the coating of which ha been oxidized to form said insulating layers on the opposite faces of said electrode.

References Cited UNITED STATES PATENTS 2,603,550 7/ 1952 Bloomsburgh 315-31 X 2,919,381 12/1959 Glaser BIS-31 3,031,597 4/ 1962 Davis 313-85 X RODNEY D. BENNETT, Primary Examiner.

M. F. HUBLER, Assistant Examiner. 

1. IN A POST DEFLECTION FOCUS CATHODE RAY TUBE HAVING AN ENVELOPE WITH A CONICAL SECTION LUMINATING IN AN IMAGE SCREEN AT ONE END AND HAVING AT LEAST ONE ELECTRON GUN AT THE OPPOSITE END FOR DEVELOPING AND DIRECTING AN ELECTRON BEAM ALONG A PREDETERMINED PATH AND THROUGH A BEAM DEFLECTION REGION TOWARD SAID IMAGE SCREEN, THE IMPROVEMENT WHICH COMPRISES: A UNITARY ELECTRODE STRUCTURE POSITIONED ACROSS SAID BEAM PATH BETWEEN SAID SCREEN AND SAID DEFLECTION REGION AND HAVING A PRINCIPAL CONDUCTIVE MEMBER WITH A MULTIPLICITY OF ELECTRON PERMEABLE PORTIONS AND INTERVENING ELECTRON IMPERVIOUS PORTIONS; A PAIR OF AUXILIARY CONDUCTIVE MEMBERS SUPERPOSED OVER BUT INSULATED FROM BOTH FACES OF SAID ELECTRON IMPERVIOUS PORTIONS OF SAID PRINCIPAL MEMBER; MEANS FOR APPLYING OPERATING POTENTIALS TO SAID AUXILIARY MEMBERS; AND MEANS FOR APPLYING A DIFFERENT OPERATING POTENTIAL TO SAID PRINCIPAL MEMBER TO FORM AT EACH OF SAID ELECTRON PERMEABLE PORTIONS A LENS FOR FOCUSING SAID BEAM ON SAID SCREEN. 